A tetrachord a group of four notes arranged whole step, whole step, half step and written on four adjacent lines and spaces on the staff is a four note pattern consisting of whole step an interval of two half steps, a tone - whole step - half step a semitone, the octave is divided into twelve half steps. A whole step consists of two half steps. This diagram shows a tetrachord on a single string.
Above is a tetrachord on a string instrument and below on the piano keyboard.
A Major scale a group of pitches arranged from high to low covering one octave uses seven of the twelve notes within the octave an interval with the same note names but twelve half steps higher or lower. It can be built with two tetrachords separated by a whole step.
Here is a Major scale showing tetrachords on the piano:
Even though there are twelve notes within the octave, our system of notation is built around the seven note scale. It uses seven letters of the alphabet: ABCDEFG.
To construct a Major scale on any starting pitch, construct two tetrachords separated by a whole step. Use the Instrument Scales by Eye and Ear objectives in MusicGoals to master tetrachords and scales.
see also: Keyboard Scales, String Scales